Butterfly valves are extensively used in the bleed air, pneumatic and air conditioning systems of most modern aircraft. In the aircraft industry, seal rings are frequently used in butterfly valves that are typically not very reliable due to the nature of the environment in which they must operate. Most butterfly valve applications demand good leakage control at high and low pressures as well as optimum performance over a large range of operating temperatures.
Butterfly valves typically include a valve housing within which is disposed a rotating main butterfly plate. A seal ring is seated in a seal ring groove that is provided in the periphery of the plate. As the plate rotates between open and closed positions, the seal ring engages the inside surface of the valve housing to seal the edges of the plate with respect to the valve housing. Proper installation of the seal ring/butterfly plate combination inside the bore of the valve housing is critical since the plate must be perfectly aligned to the bore of the valve body to allow proper butterfly plate rotation. This assembly process is time-consuming and increases the labor hours during valve overhaul.
A typical aircraft pneumatic system includes several butterfly valves to control bleed air regulation and shut-off. Butterfly valves accomplish this function by regulating or completely shutting off the airflow or other fluid flow in a given duct system. When the butterfly valve functions as a regulating valve, the main butterfly plate must oscillate between open and closed positions during flight. This regulation is used to increase or decrease the demand of air for the system and simultaneously maintain a constant pressure within the duct. When the butterfly valve functions as a shutoff valve, the main butterfly plate is used to completely open the air flow or completely block air flow in a desired section of the duct system. In this case, leakage control across the butterfly plate becomes very important. If a valve fails, depending on its location, it can render the bleed system and its accessories inoperative.
As the main butterfly plate rotates in the valve housing, pressure acting on the surface of the plate causes the seal ring to move inside the seal ring groove. This movement constantly changes the orientation of the ring relative to the plate. Since the typical system has high pressure on one side and low pressure on the opposite side of the plate, if the ring ends are not secured to the plate or if the ring has a “self-locking” mechanism, pressure can cause the ring to “blow-out” of the groove. If the ring ends are deflected from their normal operating condition in the groove, as the butterfly plate moves, it can cause the valve to lock up by wedging the ring between the clearance of the outside and inside diameters of the valve body.
Manufacturers of butterfly valves utilize various methods of securing the seal ring to the main butterfly plate. The most common method is to drill slots on the side faces of the seal ring and match these slots with drilled holes on the side wall of the plate. A wire sized to fit into these holes is driven across both the plate and the ring. The typical shape of this wire resembles a staple. Once this staple is installed, the ends are bent in opposite directions to ensure ring end retention. This method, while effective in securing the ring, results in high leakage across the seal/plate assembly. Most of the time, the rings have straight cut joints which are not optimally effective to control leakage.
As it wears during operation, the seal ring eventually no longer rotates relative to the plate due to the locking staple. This rotation is known as ring “walking”. As the ring is now locked in place, it starts wearing at both extremes closest to the axis of rotation of the main butterfly plate, thus further increasing leakage across the plate.
Another method of securing the seal ring to the main butterfly plate involves the use of a small-diameter dowel pin that rides inside the gap of the seal ring. This dowel pin is press-fitted into the main butterfly plate. The typical location of the pin is at the axis of rotation of the plate where ring “walking” is negligible. While this solution appears to be simple, it also has disadvantages. The major disadvantage of the method is looseness of the press fit of the pin. If the pin is lost, it can travel downstream of the valve, causing damage to other components of the system or to the valve itself. Loosing the pin will cause the ring to walk; thus, ring blow-out is eminent. As with the staple, this method also causes the ring to wear at the areas closest to the axis of rotation of the butterfly plate, thus causing leakage to increase.
One of the most effective methods of ring retention involves the use of a self-locking ring. An example of such a self-locking ring is the self-locking seal ring available from the Kaydon Corporation of Muskegon, Mich. The Kaydon Corp. self-locking seal ring has a latch structure of overlapping, interengaging surfaces inclined to the ring's axis. The inclined surfaces seat against each other and intersect only the outer circumferential face of the ring and one of the axial faces of the ring at a location spaced radially outwardly from the inner radial face of the ring. The axial opening through the ring at the center of the overlap is entirely surrounded by portions of the outer circumferential face of the ring.
Due to the intricate shape of the locking feature of the Kaydon Corp. self-locking ring, the ring cannot be installed inside a normal machined groove. Instead, the butterfly plate must have a removable sector to allow its installation and proper “hooking” action between the ends of the ring. The ring is free to “walk” in the groove during valve operation, thus distributing wear around its circumference. It has been found that leakage control of the ring is better than previous cases, although the biggest disadvantage of the ring is the special provisions the butterfly plate must have to facilitate its installation. For example, the removable sector must be manufactured separately from the rest of the plate. During assembly, the plate must match perfectly to the contour of the rest of the plate to allow smooth plate rotation. Additionally, provisions must be made to control leakage at the interfacing surfaces of the removable sector with the butterfly plate. Assembly procedures become cumbersome, time-consuming and expensive.
Another example of a self-locking seal ring is manufactured by Cook Airtomic, a division of the Dover Corporation. The locking mechanism of this seal ring is shown in FIG. 9 of the drawings. The seal ring is known as a “double-hook joint ring” or PDS ring. Like the designs of other types of seal rings used to seal butterfly valves, this seal ring design requires special considerations during its installation, although testing and service life have shown this seal ring to be very effective and efficient. However, a common drawback of the seal ring is the difficult installation of the ring into the ring groove in a butterfly valve housing. A Marcel spring is sometimes used to keep the ring centered once installed in the ring groove. Sometimes, a special installation tool is required to install the ring.
There is therefore an established need for a self-locking seal ring which is easy to install in a ring groove of a main butterfly plate and facilitates optimum sealing of the butterfly plate with respect to the butterfly valve housing.